1. Technical Field
The present invention relates generally to the field of electronic assembly and more specifically, but not exclusively, to the manufacture and assembly of electronic products without the use of solder.
2. Background Art
Historically, most electronic products have been assembled using a solder material and a soldering process. This has always had certain disadvantages, and a number of new trends are revealing or exacerbating other disadvantages.
One set of disadvantages relates to solder materials themselves. Since the earliest days of the electronics industry tin/lead type solders (e.g., Sn63/Pb37) have been widely used. Unfortunately, both tin and especially lead have serious chemical disadvantages. For these two metals, mining the ores, refining it, working with the refined metals during manufacturing, being exposed to the substances in manufactured products, and disposing of the products at the ends of their life cycles are all potentially damaging to human health and to the environment.
Recently human health and environmental concerns about tin/lead type solders have resulted in the Directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (commonly referred to as the Restriction of Hazardous Substances Directive or RoHS) in the European Union. This directive restricts the use of six hazardous materials, including lead, in the manufacture of various types of electronic and electrical equipment. This directive is also closely linked with the Waste Electrical and Electronic Equipment Directive (WEEE) 2002/96/EC, which sets collection, recycling, and recovery targets for electrical goods. Together these directives are part of a growing world-wide legislative initiative to solve the problem of electronic device waste.
To some extent the electronics industry has always been searching for a practical substitute for tin/lead type solders, and legislative initiatives like those just noted are now motivating a number of changes. Today a common substitute for tin/lead type solders are SAC type solder varieties, which are alloys containing tin (Sn), silver (Ag), and copper (Cu). But this is merely a compromise. Mining, refining, working during manufacturing, exposure from manufactured products, and disposal are still all issues for tin, silver, and copper. It therefore suffices here to say that the undue use of some materials, like solder, is generally undesirable in electronic assemblies.
Another set of disadvantages in the solder-based assembly of electronic products is the high temperature processes that are inherently required. The use of heat on and around many electronic components has always been undesirable. As a general principle, the heating of electronic components increases their failure rate and beyond a certain point outright damages such components. Tin/lead solders melt at relatively low temperatures, and their use has generally been tolerable for many components. This is not as frequently the same for SAC type solders, however, which melt a much higher temperatures (e.g., ˜40° C. or greater). When SAC type solders are used the likelihood of component damage is much higher, resulting in assemblies that fail during post-manufacturing testing as well as in-the-field failures. Additionally, generating and managing the heat during manufacturing have energy, safety, and other increased costs. It therefore further suffices here to say that the undue use of head-based manufacturing processes, like soldering, is also generally undesirable in electronic assemblies.
Increasingly yet another set of disadvantages in the solder-based assembly of electronic products is related to the “adding” of materials. When a material, like solder, is added between two components to hold them together the additional material inherently has to occupy some space. The use of liquid-state materials, like solder in its liquid stage, in manufacturing also often requires additional space around leads, terminals, and pads because both product and process designs need to account for the ability of liquid to flow easily and thus to potentially short to other leads, terminals, pads, etc. Liquid surface tension effects are also usually a major consideration in such designs, an liquid solders have high surface tensions. These all thus are factors as designers increasingly strive to miniaturize electronic assemblies. And it therefore yet further suffices here to say that the undue use of additional material in manufactured assemblies and manufacturing processes, again like solder, is generally undesirable in the resulting electronic assemblies. The following figures will help to illustrate some of these points.
FIG. 1 (prior art) is a side cross-section view of a conventional solder-based assembly 100 including a printed circuit board (PCB 102) and a component package 104. The package 104 includes an electrical component 106 having gull wing terminals or leads 108 (one lead 108 shown). A solder joint 110 connects the lead 108 to a terminal pad 112 on the PCB 102. Elsewhere on the PCB 102 insulating material 114 prevents material from the solder joint 110 flowing to where it can short to other solder joints, leads, or terminal pads. In the particular example shown, the PCB 102 is a multi-layer type where a conductor filled through hole 116 connects the terminal pad 112 to one or more conductive traces 118.
This prior art approach has a number of disadvantages. It uses solder, which may contain undesirable materials (e.g., lead) and which requires heat-based manufacturing processes (i.e., soldering). As implied above, solder from the solder joint 110 can flow, thus motivating the use of the insulating material 114 and various additional design concerns to avoid having the solder joint 110 short to other solder joints, leads, or terminal pads. This also complicates both the external structure of PCB 102 as well as the internal structure of the PCB 102. Furthermore, since the solder at the solder joint 100 inherently occupies some space, its presence increases the height of the overall assembly 100.
FIG. 2 (prior art) is a side cross-section view of another conventional solder-based assembly 200 including a printed circuit board (PCB 202) and a different component package 204. The package 204 here includes an electrical component 206, leads 208 (one lead 208 shown), a supports 210, and an insulating base 212. The PCB 202 here includes a terminal pad 214, an insulating material 216, and a through hole 218 connecting the terminal pad 214 to one or more conductive traces 220.
Here a ball of solder 222 connects the lead 208 to the terminal pad 214. But in most respects, particularly including the disadvantages already discussed for the assembly 100, the assembly 200 is largely the same.
FIG. 3 (prior art) is a side cross-section view of a solderless connection apparatus 300 in accord with U.S. Pat. No. 6,160,714 by Green. In this configuration, a substrate 302 supports a component package 304. The package 304 contains an electrical component (not shown) such as an IC or other discrete component. Overlying the substrate 302 is an insulating material 306. On the other side of the substrate 302 is a conductive, polymer-thick-film ink 308. To improve conductivity, a thin film of copper plating 310 is provided on the polymer-thick-film ink 308. A via extends from the package 304 through the substrate 302. The via is filled with a conductive adhesive 314. The point of attachment 316 of the package 304 to the adhesive 314 may be made with fusible polymer-thick-film ink, silver polymer-thick-film conductive ink, or commercial solder paste. One disadvantage of this prior art apparatus 300 is the additional thickness added by the adhesive 314 as illustrated by the bump 318.